1. Field of the Invention
The present invention relates to a diffractive optical element to be used for an optical apparatus such as a camera or a video camera, and a manufacturing method for the same.
2. Description of the Related Art
A diffractive optical element typified by a diffraction grating includes multiple grating portions arranged in a concentric manner, and each of the grating portions has an optical effective surface for forming an image of incident light at a desired position and a grating wall surface that does not work as an optical element. If light from outside of an effective screen enters the grating wall surface, the light may reach a position shifted from a desired position of image formation and become flare. If there is a lot of flare, image quality is largely deteriorated. In order to suppress the flare, it is effective to form a light shielding layer on the grating wall surface, and there are proposed various methods.
U.S. Pat. No. 5,676,804 discloses a technology involving forming a primary film only on the optical effective surface, forming a film made of a light shielding material on the entire surface of the grating, and then using a lift-off method so that the light shielding layer remains only on the grating wall surface where the primary film has not been formed.
In addition, Japanese Patent Application Laid-Open No. H11-251215 proposes, as one of steps of a method of forming a resist on a stepped portion, a method of forming an antireflection film only on the grating wall surface. Specifically, the method includes forming a uniform film of an antireflection material on the entire surface of the substrate, and then performing etch back of the entire surface of the substrate by reactive ion etching so that the antireflection film remains only on the grating wall surface.
According to the above-mentioned conventional technologies, it is possible to form the light shielding layer on the grating wall surface, but there are problems as follows. The method disclosed in U.S. Pat. No. 5,676,804 is a vapor deposition method having strong directivity in which a film forming angle is optimized and devised so that the film is formed only on the optical effective surface, but actually, wraparound to the grating wall surface cannot be suppressed. The primary film formed on the grating wall surface has a density lower than that of the primary film formed on the optical effective surface. In this state, if the light shielding material is formed on the entire optical element, a light shielding film on the grating wall surface becomes a low density film imitating the underlayer. A state of the low density film formed on the grating wall surface changes largely depending on an angle between the grating wall surface and the grating slope. If the angle is approximately 80 degrees, film density of the low density film becomes approximately ¼ to ⅓ of that of a high density film. Further, when the lift-off process is performed, there occurs a problem that the light shielding layer that must remain on the grating wall surface is partially removed. If the density of the light shielding film is low, light shielding ability as a film is decreased, and hence a sufficient effect for the flare suppression cannot be obtained.
Further, there is another problem in that a region where the light shielding layer is not formed occurs on the grating wall surface. This is caused by the fact that the film made of a light shielding material is not formed near the lower end portion of the grating wall surface because the light shielding material is formed in the state in which the primary film is formed on the entire optical effective surface. If the region where the light shielding layer is not formed occurs on the grating wall surface, the flare suppression is not performed on the region, and hence a sufficient effect for the flare suppression cannot be obtained as a whole.
In Japanese Patent Application Laid-Open No. H11-251215, anisotropic etching is performed by oxygen reactive ion etching (RIE) so as to attempt to remove only the film of the flat portion. However, because the film having uniform density and thickness is formed on the entire surface, a contrast of etching rate between the flat portion and the grating wall surface cannot be obtained sufficiently. As a result, when etching the flat portion, the grating wall surface is also etched and the film of the grating wall surface becomes a low density film, and hence a desired function of the film cannot be obtained sufficiently. As a specific problem in a case of a diffraction grating, light shielding performance is insufficient so that a sufficient effect for the flare suppression cannot be obtained. On the contrary, if the etching is stopped in the state in which sufficient light shielding performance can be obtained in the film of the grating wall surface, the film remains also on the optical effective surface, and hence a problem such as a decrease in transmittance occurs.
Further, if the material to be etched is an organic film, it is easy to form a wall surface protection film that is important for the anisotropic etching. However, if the material to be etched is an inorganic material, the wall surface protection film is formed less because the etching rate is low. Therefore, there is also a problem in that when the inorganic material is etched, etching rate contrast cannot be secured compared with the case of the organic material.